wakeech

well, all in all, it amounts to "not the fuel"... good enough for me.

and ya, rpm_power, your reasoning seems correct (on the more powerful with the lower octane)... i just wonder how much of a difference it would make...

R1_Kilroy

This is actually a common falacy. Higher octane fuel does not have a slower burn. It has more resistance to pre-ignition. Octane rating and burn speed are not related. Additives can be added to fuel that increases the burn speed without affecting octane. Nutec is a race fuel that, while extreamly corrosive, has a very fast burn rate - the faster burn rate will give an extra 10% power.

Don

BlueAdept

OK, but why would the timing have any way to control problems caused by too low octane fuel? If it's only pre-ignition that's the problem, why would sparking later make a difference?

Macabre

pre-ignition cannot be cured by timing retard, but detonation often can. Pre-ignition is when a hot-spot ignites the mixture before the spark fires. Detonation is caused by uncontrolled combustion burning unevenly and hitting everything in the combustion chamber with a pressure wave from the explosion. By lighting the spark later, the fuel/air is more evenly compressed when it's ignited and burns in a controlled fashion that can by properly absorbed by the piston or rotor. The downside of timing retard is it's still burning while the piston or rotor is on its way into the exhaust cycle so some of the energy is wasted. Fuel with a higher anti-knock index is less susceptable to either because it's volatility is lower.

BlueAdept

OK, I understand pre-ignition... and incorrect timing... I've tuned a lot of engines, so I know that noise well! But "Detonation" is not somthing I've come across... perhaps becuase most cars I've owned ran on 98+ octane even the lowest grade we get is 95...

I understand the concept pretty well, being an engineer at heart, but can you (anyone) explain what causes detonation, and what that feels/sounds like... hopefully how to avoid it should be pretty self explanatory from there!

BlueAdept

Do you, or anyone here know what it takes to reset an ECU?... or otherwise what it would take for the ECU to learn that you now have better fuel than you did yesterday...

fritts

On most vehicles it just a matter of unhooking the positive terminal of the battery and rehooking it up.

R1_Kilroy

First off, let me say everything I know relates to reciprocating piston engines.

Now.

Detonation and pre-ignition are different terms for the same thing. Both mean ignition of the fuel/air mixture prior to the spark-plug initiated flame reaching the that portion of the mixture.

You asked a great question - why does retarding the ignition solve the problem. This is counter-intuitive - what good does it do to spark the mixture later when it is already igniting before the spark.

The answer is fairly complicated so bear with me...

Ping, pre-ignition or detonation occurs under high-load circumstances. This can be translated as low revs, high gear and/or ascending a hill working against gravity. This type of situation results in very high combustion pressures (imagine starting a bicycle in the highest gear - it is very difficult to pedal and you really have to work hard).

This high combustion pressure causes extreme heat in the combusion chamber. These "hot spots" cause pre-ignition of the mixture.

For the sake of argument, imagine a combustion chamber as a clock with the spark plug at the 12 o'clock position. Imagine piston Top Dead Center (TDC) as the 10 and 2 position. The reason you get more power from spark advance is the flame takes time to start at 12, move to the 11 and 1 position, then the 10 and 2, then 9 and 3 and so on.

If you advance the spark ignition to fire when the piston is at the 9 and 3 position, the explosion should arrive at the TDC position sooner than it would have if it fired at the TDC position. This results in more power. Obviously, if you advance the timing too much you can have the explosion hitting the piston crown as it is still ascending. This can result in, most comonly, pitted pistons and broken piston-ring landings.

Now, why does retarding the ignition timing reduce knocking. It has to do with leverage (the charge has an easier time pushing down the piston once it has started to descend) and the fuel/air charge sits in the combusion chamber longer, providing a cooling effect. Downshifting when you hear knock will raise your engine revs (less load) and increases your mechanical gearing advantage (less load).

Remember, the knock sensor can only vary ignition a few degrees. So just because a vehicle has one, does not mean it will not suffer from pre-ignition. This situation is critical with turbo-charged cars.

Now, I am out of breath!

Ciao,

Don

Macabre

"It is the impact of the resulting shock waves on the engine's parts that produces the metallic ping or knock that is heard. To sum up, we can say that the treatment of the unburned charge causes changes in its nature, possibly resulting in auto-ignition."

long winded description: http://link.sandiego.com/scripts/whe...idc?passin=297

ptiemann

Just for the record, I heard my first pinging yesterday. At a speed of ~ 84 mph, 6th gear on a flat freeway. Going off the gas and keeping it @ 80 mph, the ping was gone. Back up to 84, the ping came back.

The sound is very low and I wouldn't have noticed it if the radio had been on.

I've driven faster than that before and didn't hear a ping. Up there it is explained that low revs (= higher load on engine) could cause it. Seems opposite to what I observed.
Anyway.

I attribute it to the fact that I was running cheap Costco 87 gas. I had run out of gas 2 days before (road trip) and filled up - emergency in unknown territory - at Costco <rolling eyes>. Normally I buy 87 octane gas from Union 76 or Chevron.

As the pinging occurred, I then stayed at 80. Since my tank was almost empty anyway, I filled up at the next station - Chevron 93 octane.

Further testing didn't produce any pings at speeds up to 110 mph.